1. Field of the Invention
The present invention relates to a silicon-titanium mixed oxide powder that can be produced by flame hydrolysis, and to the production and the use of the powder.
2. Discussion of the Background
It is known to produce silicon-titanium mixed oxide powders by flame hydrolysis. This generally involves hydrolyzing a mixture of silicon tetrachloride and titanium tetrachloride in a flame. The flame may be produced, for example, by the reaction of hydrogen and atmospheric oxygen, which gives rise to the water necessary for hydrolysis of the chlorides. The reaction products obtained are the silicon-titanium mixed oxide powder and hydrochloric acid, some of which remains attached to the powder.
However, the processes described in the prior art only give rise to powders with a limited TiO2/SiO2 ratio.
DE-A-2931810 accordingly claims a silicon-titanium mixed oxide powder which contains 0.1 to 9.9 wt. % titanium dioxide. It is produced by vaporizing silicon tetrachloride, diluting it with preheated air and mixing it in a mixing chamber with hydrogen and titanium tetrachloride and combusting the mixture in a reaction chamber.
DE-A-4235996 claims a silicon-titanium mixed oxide powder which contains 70 to 99 wt. % titanium dioxide. It is produced by vaporizing silicon tetrachloride and transferring it into a mixing chamber by means of an inert gas, where it is mixed with hydrogen, air and titanium tetrachloride and the mixture is combusted in a reaction chamber.
However, it has not been possible with either of these processes to obtain a silicon-titanium mixed oxide powder outside the indicated limits of titanium dioxide. When such attempts have been made, dark colored impurities and coarse particles were often obtained.